The present invention relates to an automatic injector for injecting one or more injection liquids.
Such an injector, viz. for injecting two or more different injection liquids, is known from U.S. Pat. No. 4,529,403. The injection needle of the injector described in that patent comprises a needle sheath of a flexible material that maintains the needle in a sterile condition during storage of the injector and serves as a shock absorber for the cartridge during operation of the injector. Such a needle guard is an excellent provision and is preferably also used in the injector according to the present invention.
Automatic injectors have been designed especially for use by human beings who have to administer an injection into their own body at a given instant that is not known beforehand. These persons include, for example, military persons exposed to an enemy's battle gas, for example, a nerve gas. Consequently, automatic injectors must satisfy stringent requirements regarding their reliability. Such injectors are usually stored for years at a time and in addition, after having been transferred to the potential users, are kept by those users for a long period of time under varying conditions. The proper operation of the injectors must be sufficiently ensured. At the critical instant, in fact, the lives of the users may depend on the operation of the injectors. Automatic injectors therefore must satisfy stringent requirements for storage stability. In particular, military authorities in various countries require a storage stability of at least five years, i.e. after five years of storage the automatic injectors must still operate reliably.
The injector described in U.S. Pat. No. 4,529,403, in particular the embodiment shown in FIGS. 1 and 2, has the disadvantage that during storage the foremost injection liquid, i.e. the injection liquid in front of the foremost separating stopper, is in constant contact with the synthetic material of the needle holder and with the metal of the injection needle. As a result of this, the application possibilities of the foremost compartment, i.e. the space in the needle holder, is restricted, because liquids present therein must be capable of withstanding contact with synthetic material and metals for at least five years. Of course, this is a restriction on the possible uses of the injector. An obvious solution, namely omit the injection liquid from the foremost compartment, in fact is not a solution. If such an injector were used, the air present in the foremost compartment would also be injected. This would be a severe disadvantage, because the air would enter the blood stream of the user and could cause an air embolism. It is therefore desirable to minimize the amount of air injected together with the injection liquid. One possibility for this purpose would be to reduce the length of the sealing stopper and hence the space in the needle holder. However, a thinner sealing stopper involves the risk of leakage and evaporation of the injection liquid, especially in the case of a required storage period of five years, so that a thinner stopper is not an acceptable solution to the problem.